This new R03 research proposal addresses PA-96-048 entitled Expanded Research on Emerging Diseases St. Louis encephalitis (SLE) virus has been consistently active in California since the 1930's and may re- emerge in the future as a serious health threat. This virus poses an increasing threat to California as wetlands are restored, housing developments and recreational sites located adjacent to them, and the use of pesticides increasingly restricted. In seasons when mosquitoes are active, SLE virus is maintained in enzootic cycle involving wild birds and mosquito vectors. We proposed that SLE virus is maintained through adverse seasons and years in California by persistence in enzootic foci with periodic emergence of new genotypes and/or periodic introduction from other enzootic areas. The objective of the proposed research is to use a molecular genetic approach to address the recrudescence and dispersal of SLE virus. Phylogenetic analysis of nucleotide and amino acid sequences of SLE viral isolates from throughout California and a comparison with viral isolates from other sections of the United States including the Mississippi River Basin, East Coast and Texas, and the Caribbean, Panama, and South and Central America, over an extended period of time, will allow determination of the rate at which the SLE viral genotype is changing, whether there is constant molecular change or episodes of accelerated change, and whether the virus is being carried between discrete geographic areas. Proposed research tests the hypothesis that isolates of SLE virus in given geographic areas show little genetic change over time, suggesting that they persist in those areas, but new genotypes are periodically introduced or emerge which circulate with the previously existing genotype. This study may help explain one means of emergence of flaviviruses from enzootic foci in temperate climates.